They say one injured person remained trapped in a vehicle Thursday evening, hours after the pileup began.

The accident happened near Michigan City, which is about a 60-mile drive from Chicago. Police say Michigan City has provided two buses that are being used to warm people and transport the injured to hospitals.

"We removed vehicles and accessed some people that have been previously trapped. We're addressing those as we come to those, we have no idea how many others may still be located somewhere in the wreckage," Lt. Peter Wood of the Indiana State Police told ABC affiliate WLS. "Right now the challenges are the cold and the darkness."

Photos of the scene show at least a dozen semitrailers jammed together the width of the highway near an overpass. Some passenger cars were sandwiched in the wreckage.